1. Field of the Invention
The present invention relates to an organic electroluminescent display device.
2. Description of the Related Art
In recent years, image display devices including a self-luminous element called an organic light-emitting diode (OLED), hereinafter simply “organic electroluminescent (EL) display devices”, have been in practical use. Such an organic EL display device with a self-luminous element is excellent in visibility and response speed compared with liquid crystal display devices known in the art. Additionally, the organic EL display device eliminates the need for an auxiliary lighting device, such as a backlight, and thus can be made thinner.
There are two major types of color display methods in such organic EL display devices. In one method, each pixel has red (R), green (G), and blue (B) light-emitting elements that emit light of three colors: R, G, and B, respectively. In the other method, each pixel has a white light-emitting element and R, G, and B color filters, which respectively allow passage of light in the wavelength ranges corresponding to the three colors R, G, and B. There are also two types of light-emitting elements. One of them has only a single light-emitting layer. The other, called a tandem light-emitting element, has a stack of light-emitting layers and a charge generation layer (CGL) between them to activate each of the light-emitting layers to emit light.
JP 2006-049396 A discloses that an electron pull-out layer (A) is disposed between an anode electrode and a hole transport layer (B) and that the energy level difference between the highest occupied molecular orbital of the hole transport layer, HOMO (B), and the lowest unoccupied molecular orbital (LUMO) of the electron pull-out layer, LUMO (A), is expressed as 0<LUMO (A)−HOMO (B)<1.5 eV. JP 2009-301885 A discloses a multi-unit organic EL element having light-emitting layers and conductive intermediate layers laminated alternately, in which the conductive intermediate layers include at least a transparent conductive carbon layer.
When used in each pixel of the above organic EL display device, an organic EL element driven at a high voltage increases the power consumption of the organic EL display device. Moreover, such an organic EL element accelerates its deterioration to shorten the useful life of the organic EL display device.